The present disclosure relates to a developing device including a stirring member that stirs and conveys a developer and to an image forming apparatus.
In general, an electrophotographic image forming apparatus includes a developing device. One-component development schemes and two-component development schemes are known as development schemes using a dry toner that are employed in generic developing devices. In the one-component development schemes, a one-component developer consisting only of a magnetic toner is used. In the two-component development schemes, a two-component developer including a non-magnetic toner and a magnetic carrier for charging the toner is used, and an electrostatic latent image on an image bearing member (photosensitive member) is developed with a magnetic brush that is formed on a developing roller from the magnetic carrier and the toner.
In a generic developing device, toner is consumed during development. The developing device is therefore provided with a toner sensor that detects a toner concentration or a toner amount, and toner is newly supplied in an amount that makes up the consumed amount. Image defects such as toner scattering and fogging may occur if the newly supplied toner is not sufficiently charged. In the two-component development schemes, therefore, the toner and the carrier need to be sufficiently stirred and mixed to charge the toner to a desired charge. Likewise, in the one-component development schemes, the toner remaining in the developing device and the newly supplied toner need to be sufficiently mixed to give a uniform charge distribution.
To this end, a widely-known method involves conveying a developer by circulating the developer through a developer circulation pathway provided within a developing device while stirring and mixing the developer using a stirring conveyance member disposed in the circulation pathway. The stirring conveyance member has a shape of a screw including a rotational shaft and a helical blade. More specifically, an inner space of a developer container is divided by a partition wall into two developer conveyance paths elongated in a longitudinal direction of the developer container. Each of the two developer conveyance paths includes one stirring conveyance member. The two developer conveyance paths and two developer transfer sections form the developer circulation pathway. One of the developer transfer sections allows communication between one end of one developer conveyance path and one end of the other developer conveyance path. The other of the developer transfer sections provides communication between the other end of the one developer conveyance path and the other end of the other developer conveyance path.
Requirements to increase image formation speed and to decrease the size of image forming apparatuses have necessitated increase in circulation speed (i.e., conveyance speed) of a developer in a developing device. Examples of common methods of increasing the developer conveyance speed include a method involving increasing the rotational speed of stirring conveyance members, and a method involving widening the helical pitch of helical blades.
Such common methods increase the developer conveyance speed in an axial direction of stirring conveyance members. However, the conveyance speed is not significantly increased at communication sections (i.e., developer transfer sections) formed at turn-around sections of a circulation pathway. Consequently, the developer accumulates at the developer transfer sections, and therefore may be packed into an aggregation or may adhere to an inner wall of a developer container.
To solve such a problem, methods have been proposed for reducing concentration of a developer at developer transfer sections. For example, a developing device has a plate-like paddling fin disposed at a downstream end of a stirring conveyance member in terms of a developer conveyance direction to improve the developer conveyance ability at a developer transfer section. The paddling fin extends outward in a radial direction of the stirring conveyance member so as to be opposite to the developer transfer section.